1. Field of the Invention
The present invention relates to a fixing device or the like to be used in an image forming apparatus utilizing the electronic photography and, more particularly, to a fixing device or the like equipped with a turnable belt member.
2. Description of the Related Art
In the image forming apparatus such as a copier or a printer using the electronic photography, the image formation is performed, as follows. First of all, the surface of a photosensitive element (or a photosensitive drum) formed in a drum shape, for example, is homogeneously charged by a charging device. The photosensitive drum charged is scanned with and exposed to a beam controlled on the basis of image information so that an electrostatic latent image is formed on the surface of the photosensitive drum. Subsequently, the electrostatic image formed on the photosensitive drum is visualized (into a toner image) by a developing device, and the toner image is then conveyed to a transfer unit, as the photosensitive drum rotates, so that it is electrostatically transferred to a sheet of recording paper. The toner image on the recording paper sheet is subjected to a fixing treatment by the fixing device so that the toner image is completed.
As the fixing device used in such image forming apparatus, a structure called the “two-roll type” has been widely utilized in the related art. This two-roll type fixing device is configured such that a fixing roll formed by laminating a refractory elastic layer and a peeling layer on the surface of a cylindrical core having a heat source (or a heater) therein and a pressure roll formed by laminating a core and a peeling layer of a refractory resin coating film or a refractory rubber coating film are pressed to contact with each other. The toner image is fixed by passing the recording paper sheet carrying the unfixed toner image through the pressed contact area (or the nip portion) between the fixing roll and the pressure roll thereby to heat and press the unfixed toner image.
In recent years, many image forming apparatus have spread as the mass production or the coloring of the apparatus has abruptly developed. Therefore, the fixing device mounted on the image forming apparatus also has to be developed more for the high speed.
However, the two-roll type fixing device of the related art has a problem that a sufficient fixing treatment is difficult for a number of sheets of recording paper being continuously sent at a high speed. In the two-roll type fixing device, more specifically, the core composing the fixing roll and the elastic layer of silicone rubber or the like coating the core act as thermal resistors. As a result, the two-roll type fixing device has found it structurally difficult to feed such a calorie instantly and sufficiently from the heater arranged in the fixing roll as corresponds to the calorie to be rubbed by the recording paper sheets from the surface of the fixing roll.
As a result, if the two-roll type fixing device is continuously fed at the high speed with the recording paper sheets, the surface temperature of the fixing roll gradually lowers to cause a disadvantage that the fixing performance is gradually deteriorated. At the rising time of the image forming apparatus, moreover, there easily arises the “temperature drooping phenomenon”, in which the surface temperature of the fixing roll temporarily drops. Especially in case thick sheets of paper or the like of a high heat capacity are employed as the recording paper sheets, the calorie to be rubbed from the surface of the fixing roll increases to lower the fixing performance and to enlarge the temperature droop with the result that the image quality is deteriorated by the fixing failure.
In these situations, there has been developed a technique for realizing a fixing device matching the high speed of the image forming apparatus by solving the aforementioned problem which is caused in the case of using the two-roll type fixing device. For example, one technique (as referred to JP-A-3-133871, for example) resides in the fixing device, in which a heating member for heating the recording paper sheets is made of a film-shaped belt member (or a fixing belt) extended by plural tension rolls.
In this fixing device using such fixing belt, the toner image is fixed by heating the fixing belt sufficiently in advance with the heaters arranged in the tension rolls before the fixing belt enters the nip portion, thereby to apply the heat to the recording paper sheets and the toner image at the nip portion from the fixing belt heated. Even if, therefore, the fixing belt is robbed of the heat by the recording paper sheets during the fixing treatment, the fixing belt is enabled to restore a predetermined fixable temperature for a short time period by the heaters in the tension rolls, because the heat capacity of the fixing belt itself is small. In the fixing device using the fixing belt as the heating member, therefore, it is easy to keep the temperature of the fixing belt at the predetermined value when the fixing belt enters the nip portion, and it is possible to feed a sufficient calorie to the nip portion even if the image forming apparatus is speeded up.
In the fixing device using the fixing belt, however, the toner image is carried on the surface of the recording paper sheet, so that the toner image becomes an adhesive to establish a sticking force between the paper sheet and the fixing belt when the toner image is melted by the heat of the fixing belt. This makes it necessary as in the two-roll type fixing device of the related art to provide a mechanism for peeling the paper sheet off the surface of the fixing belt. Especially in case the image forming apparatus is speeded up, when a peeling failure once occurs in the fixing device to cause a paper clogging (or a jamming), many succeeding paper sheets are damaged by the influence of the jamming. This makes it necessary to peel the recording paper sheet having passed the nip portion at the high speed, stably and reliably off the side of the fixing belt.
The mechanism for peeling the paper sheet off the fixing belt surface is configured in the related art such that a peeling pawl is arranged to abut against the fixing belt on the downstream side of the nip portion, as described in JP-A-3-133871. In the fixing device having the configuration, in which a pressure roll is arranged and pressed to contact with the fixing belt looped over the fixing roll and the heating roll under tension, on the other hand, there is used the configuration (as referred to JP-A-2003-5566, for example), in which a fixing member for setting the curvature of the fixing belt at the exit portion (i.e., the most downstream) of the nip portion is disposed on the inner side of the fixing belt at the position corresponding to that exit portion, so that the recording paper sheet is peeled off by the change in the curvature of the fixing belt.
JP-A-3-133871 (page 3, FIG. 3) and JP-A-2003-5566 (pages 6 to 8, FIG. 4) are referred to as related art.
In case, however, the fixing device using the fixing belt employs the separating pawl as the mechanism for peeling the recording paper sheet off the fixing belt surface, the peeling pawl has to be arranged to abut against the fixing belt so that the paper sheet may be stably peeled off from the fixing belt side. If the peeling pawl is used for the peeling action, the fixing belt surface is easily abraded by the peeling pawl. When the abrasion by the peeling pawl occurs on the fixing belt surface, a fixing irregularity corresponding to the abrasion mark in the surface of the fixing belt may be caused on the fixed image thereby to degrade the image quality. Moreover, the offset toner may gradually deposit on the abrasion mark thereby to blot the fixed image. As the abrasion of the surface of the fixing belt proceeds, moreover, the fixing belt of the thin layer may be finally broken to damage the function of the fixing device.
In case, moreover, a fixing member for enlarging the curvature of the fixing belt is disposed as such a mechanism at the exit portion of the nip portion as to peel the recording paper sheet off the fixing belt surface, the fixing belt is pressed onto the pressure roll exclusively by the tension of the fixing belt at an intermediate nip area between the entry portion of the nip portion, in which the fixing roll and the pressure roll are pressed to contact, and the exit portion, in which the fixing member is arranged. As a result, the nip pressure is relatively low in the intermediate nip area. If the recording paper sheet or the toner is heated in such low nip pressure area, the water content in the paper sheet may be gasified into water vapor, or the air in the toner may be thermally expanded to generate air gaps (or air bubbles). In case these air gaps occur, the air bubbles may migrate to disturb the unfixed toner, if the toner on the paper sheet in the nip portion is not completely fixed yet. As a result, the image failure such as the irregularity occurs in the fixed image thereby to cause a serious problem that the degradation of the image quality is invited.